PROJECT SUMMARY Borderline personality disorder (BPD) is a severe mental health condition that confers significant psychological and physical disability. Key features of the clinical presentation of borderline personality disorder include affect dysregulation, impulsivity and unstable relationships. While these features are classically borne out within social relationships, the neurobehavioral etiology of social decision-making in BPD has received limited investigation, despite the prominent role that interpersonal dysfunction plays in the expression of the disorder. To address this gap, the broad goal of this proposal is to detail the behavioral, computational and neural processes underlying three candidate abnormalities in social decision-making within BPD: (i) biases in risk appraisal when making decisions about social partners; (ii) hypersensitivity to prediction errors about social rewards and punishments; and, (iii) aggression-related social learning abnormalities. Thus, we combine well- characterized interactive games from decision neuroscience with functional magnetic resonance imaging to develop a sensitive and specific neuromechanistic understanding of the biological basis of interpersonal impairments in borderline PD. Specifically, we examine in BPD: (i) neural mechanisms underlying risk preferences in social and non-social decision-making (Aim 1); (ii) neural mechanisms of social reward/punishment sensitivity (Aim 2); and (iii) neural mechanisms contributing to impulsive interpersonal aggression (Aim 3).